The present invention relates generally to the abatement of contaminant laden exhaust from various wood manufacturing processes, and more particularly to a method that prevents destruction of the desired properties of the manufactured wood products during the abatement process.
The manufacture of various wood products, such as oriented strand board, medium density fiberboard, particle board, or plywood, typically requires processing under elevated temperatures to achieve certain desired properties and characteristics for the wood products. One available method for generating heat is to burn wood fuels such as bark, sander dust, hogfuel, shavings, or waste lumber, since these fuels are readily available as natural by-products of the wood manufacturing process. The heat generated by burning these wood fuels is often utilized to dry wood chips, fiber, veneer, or other types of wood selected as components of the ultimate wood product.
The wood selected for drying is often treated with resins or binders prior to the drying process. During the drying process, these resins or binders, along with other contaminants such as volatile organic compounds (VOC's), methanol, and formaldehyde are emitted by the drying wood. In addition, the wood fuel burning operation generates and emits other contaminants, such as carbon monoxide and many hazardous air pollutants (HAP's). Federal laws and regulations require that the concentration of these aforementioned contaminants in the process emissions must be reduced prior to discharge into the atmosphere.
Regenerative thermal oxidizers have heretofore been utilized to control the concentration of VOC's, methanol, formaldehyde, carbon monoxide, and HAP's which are vented to the atmosphere. In addition, electronic gravel filter beds or wet electrostatic precipitators may be utilized where removal of particulates such as a blue haze is also required.
Generally, thermal oxidizers utilize a supplementary heat source to increase the temperature of the contaminated process emissions to a level above the ignition temperature of the combustible contaminants, so as to oxidize the combustible contaminants, such as VOC's, methanol, formaldehyde, HAP's, and carbon monoxide. Regenerative thermal oxidizers recover heat remaining in the cleansed exhaust gas to increase the temperature of emissions entering the oxidizer thereby decreasing the amount of supplemental energy required to raise the emission to its ignition temperature.
However, when wood fuels are burned during the drying process, the regenerative thermal oxidizers utilized under present technology are not fully satisfactory because of the high temperatures encountered therein, which typically range from 1400.degree. F. (.apprxeq.760.degree. C.) to 1500.degree. F. (.apprxeq.815.degree. C.). More specifically, the wood fuel burning operation also produces by-products which include potassium and sodium. In addition, both the heat recovery media and the ceramic fiber insulation of the regenerative thermal oxidizer contain silica. The potassium and/or sodium from the process emissions react with the silica found in the regenerative thermal oxidizer at temperatures exceeding 1100.degree. F. to 1200.degree. F., depending upon the concentration of the potassium and/or sodium in the process emissions. As noted above, regenerative thermal oxidizers must operate at temperatures well above these threshold temperatures, thereby inherently causing the potassium and sodium to react with silica. Simply lowering the operating temperature of the thermal oxidizer does not provide a suitable solution because at temperatures below 1400.degree. F., a regenerative thermal oxidizer will not only fail to properly remove existing pollutants, but the regenerative thermal oxidizer will actually generate even more carbon monoxide and HAP's. The reaction of potassium and sodium with silica is highly undesirable and problematic because the reaction destroys the desired properties and characteristics of the manufactured wood products, and thereby diminishes the effectiveness and quality of the products.